The application of entropy for detecting behavioral responses in Japanese medaka (Oryzias latipes) exposed to different toxicants

Growing concern over environmental pollution by chemicals that may directly or indirectly impact human and ecosystem health make it increasingly necessary to establish a biological early warning system to minimize these impacts. To achieve this, simple and reliable approaches and criteria for environmental risk assessment must be developed. We propose a new approach for detecting behavioral responses to toxicants in flowing water. We conducted a series of tests to determine toxic responses of Japanese medaka (Oryzias latipes) to lethal concentrations of potassium cyanide (KCN; 10 mg L−1) and phenol (25 mg L−1). Fish movements were tracked by three‐dimensional (3D) biomonitoring under flow‐through conditions. The 3D data were used to evaluate behavioral responses in terms of swimming speed, vertical position in the water, and entropies of these parameters. Fish exposed to KCN surfaced more frequently than unexposed fish, whereas fish exposed to phenol stayed near the bottom. Maximum swimming speed significantly increased after exposure to both test chemicals, whereas median swimming speed significantly decreased after exposure. Despite large variations in swimming behavior between individual fish, toxic behavioral responses could be identified by the significant decrease in the entropy of vertical position after 10–30 min of exposure to the test chemicals. We conclude that the use of entropy can contribute to the development of a biological early warning system for detecting toxicants at early stages of pollution. To use the present approach to detect lower concentrations of toxicants, further studies should be conducted using long‐term monitoring of fish exposed to sublethal concentrations. © 2010 Wiley Periodicals, Inc. Environ Toxicol, 2010.